428 PROFESSOR TAIT ON 



solely employed in retarding the motions of projectiles, but some part of it exerted in 

 deflecting them from their course, and in twisting them in all kinds of directions from 

 their regular track ; this is a doctrine, which, notwithstanding its prodigious import to 

 the present subject, hath been hitherto entirely unknown, or unattended to ; and there- 

 fore the experiments, by which I have confirmed it, merit, I conceive, a particular 

 description ; as they are themselves too of a very singular kind." 



Eobins measured accurately, by means of thin screens placed across his range, the 

 deviation (to right or left) of successive shots fired from a gun which could be exactly 

 replaced in its normal position, after each discharge ; and found that it increased much 

 more rapidly than in simple proportion to the distance. Then he experimented success- 

 fully with a gun whose barrel was bent a little to the left near the muzzle, with the view 

 of forcing a loose-fitting bullet to rotate by making it roll on one side of the bore. The 

 bullet, of course, at first deviated a little to the left ; but this was soon got over, and it 

 then persistently curved away to the right. And he showed the effect of rotation very 

 excellently by suspending a ball by two strings twisted together, so as to give rotation 

 to it when it was made to vibrate as a pendulum. The plane of vibration rotated in the 

 same sense as did the ball. 



I have not had an opportunity of consulting, in the original, Euler's remarks on this 

 question. The following quotations are taken from a retranslation * of his German 

 version of Robins' work, but the statements they contain are so definite that the trans- 

 lator cannot be supposed to have misrepresented their meaning : — 



" The cause which Mr Robins assigns for the uncertainty of the shot cannot be the 

 true one, since we have indisputably proved, that it arises from the figure of the ball 

 only." p. 313. 



" if the ball has a progressive motion, we may, as has been already shewn, consider it 

 at rest, and the air flowing against it with the velocity of the ball's motion ; for the force 

 with which the particles of air act on the body will be the same in both cases." [Then 



follows an investigation.] " hence this proposition appears indisputably 



true ; that a perfectly spherical body which, besides its progressive motion, revolves 

 round its centre, will suffer the same resistance as if it had no such rotation. If, there- 

 fore, such a ball should receive two such motions in the cannon, yet its progressive 

 motion in the air would be the very same as if it had no rotation." pp. 315-7. 



Poisson's treatment of the subject is altogether unnecessarily prolix, and in consequence 

 not very easily understood. It is sufficient to say that, like Euler, he rejects t Robins' 

 explanation ; and that his basis of investigation of the effects of rotation on the path of 



* " The true Principles of Gunnery investigated and explained, comprehending translations of Professor Euler's 

 Observations, &c. &c." By Hugh Brown. London, 1277 (sic). 



t Poisson, in fact, says of his own results : — " Neanmoins, d'apres la composition de la formule qui exprime la 

 deviation horizontale a la distance du canon oil le boulet retombe sur le terrain, on reconnalt facilement que cette 

 deviation ne peut jamais etre qu'une tres petite fraction de la longeur de la portee ; en sorte que ce n'est pas au frottement 

 de la surface du boulet contre la couche d'air adjacente et d'inegale densite, que sont dues principalement les deviations 

 observc'i-s, ainsi que Robins et Lombard l'avait pense." Me'moire sur le Mouvement des Projectiles, &c. Comptes Rendus, 

 5 Mars, 1838, p. 288. 



